Tomography refers to the cross-sectional imaging of an object from either transmission, emission or reflection data collected from many different directions. Tomographic imaging deals with reconstructing an image from such data, commonly called projections. From a purely mathematical standpoint, a projection at a given angle is the integral of the image in the direction specified by that angle. Most of the powerful new medical imaging modalities that have been introduced during the last four decades, such as Computed Tomography (CT), Single Photon Emission Computed Tomography (SPECT), Positron Emission Tomography (PET), Magnetic Resonance Imaging (MRI) and 3D ultrasound (US), are the result of the application of tomographic principles.
It is desirable to have a system and method for reconstructing images, such as, for example, tomographic images representing the distribution of some characteristic across a sectional plane (2D mode) or a volume (3D mode) of a body under investigation, from measurements obtained by an imaging system. More particularly, it is desirable to perform an iterative reconstruction of the image in a time and space domain (hereinafter referred to as a spatial domain), where memory requirements and computations involved for the image reconstruction are reduced, while maintaining high image quality.